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Kemerlis" , Juerg Haefliger , Tycho Andersen , Marco Benatto , David Woodhouse , Khalid Aziz Subject: [RFC PATCH v7 13/16] xpfo, mm: optimize spinlock usage in xpfo_kunmap Date: Thu, 10 Jan 2019 14:09:45 -0700 Message-Id: <95b6fa40ce6c7afb4a9e58f8d747d86aa7a94177.1547153058.git.khalid.aziz@oracle.com> X-Mailer: git-send-email 2.17.1 In-Reply-To: References: In-Reply-To: References: X-Proofpoint-Virus-Version: vendor=nai engine=5900 definitions=9132 signatures=668680 X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 suspectscore=0 malwarescore=0 phishscore=0 bulkscore=0 spamscore=0 mlxscore=0 mlxlogscore=822 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1810050000 definitions=main-1901100164 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Julian Stecklina Only the xpfo_kunmap call that needs to actually unmap the page needs to be serialized. We need to be careful to handle the case, where after the atomic decrement of the mapcount, a xpfo_kmap increased the mapcount again. In this case, we can safely skip modifying the page table. Model-checked with up to 4 concurrent callers with Spin. Signed-off-by: Julian Stecklina Cc: x86@kernel.org Cc: kernel-hardening@lists.openwall.com Cc: Vasileios P. Kemerlis Cc: Juerg Haefliger Cc: Tycho Andersen Cc: Marco Benatto Cc: David Woodhouse Signed-off-by: Khalid Aziz --- mm/xpfo.c | 22 ++++++++++++---------- 1 file changed, 12 insertions(+), 10 deletions(-) diff --git a/mm/xpfo.c b/mm/xpfo.c index cbfeafc2f10f..dbf20efb0499 100644 --- a/mm/xpfo.c +++ b/mm/xpfo.c @@ -149,22 +149,24 @@ void xpfo_kunmap(void *kaddr, struct page *page) if (!PageXpfoUser(page)) return; - spin_lock(&page->xpfo_lock); - /* * The page is to be allocated back to user space, so unmap it from the * kernel, flush the TLB and tag it as a user page. */ if (atomic_dec_return(&page->xpfo_mapcount) == 0) { -#ifdef CONFIG_XPFO_DEBUG - BUG_ON(PageXpfoUnmapped(page)); -#endif - SetPageXpfoUnmapped(page); - set_kpte(kaddr, page, __pgprot(0)); - xpfo_cond_flush_kernel_tlb(page, 0); - } + spin_lock(&page->xpfo_lock); - spin_unlock(&page->xpfo_lock); + /* + * In the case, where we raced with kmap after the + * atomic_dec_return, we must not nuke the mapping. + */ + if (atomic_read(&page->xpfo_mapcount) == 0) { + SetPageXpfoUnmapped(page); + set_kpte(kaddr, page, __pgprot(0)); + xpfo_cond_flush_kernel_tlb(page, 0); + } + spin_unlock(&page->xpfo_lock); + } } EXPORT_SYMBOL(xpfo_kunmap); -- 2.17.1